It comes from a miscellany written some time before 1520 or so. I first came across it in 1995, and my heart began to race. I was involved in a medieval-themed social society at the time, and it dovetailed nicely with my brewing interests. Many people over the ensuing years have taken a shot at this. This is how I ended up interpreting the original, based on a lot of research through various sources.

Put grains in your mash vessel. Bring water to about 175 degrees. Traditionally, water was boiled and then allowed to cool until you could just see your reflection in it. When I measured this, it was 173-177 degrees, depending on the day. Add 6.75 gallons water to grains and stir well. Cover and allow to mash 1 hour. Run off wort slowly, recirculate until runnings are clear. Collect runnings and add hops. Boil 60 minutes. Strain out hops, chill, and pitch yeast. Ferment, transfer to secondary, bottle. I recommend either freezing 1/10 the wort and prime with that or priming with 1.5 oz corn sugar. This was not done historically, but I like a bit more carbonation. Bottling was done as far back as the 1500s, at least.

The end product is quite drinkable, not very bitter, not overly sweet or malty. I’ve brewed it several times over the years.

If you wish, you can add more hot water to the mash, mash 1 hour, draw off, and boil with the hops from the first boil. Some brewers would, instead, draw off this second mash, and use it as the liquor for a mash with fresh malt, then do a second mashing with water on the second mash and use that for the “small beer”.

The “malt mix” is a guess to reflect less even malk kilning available historically, but paleness was a desirable quality in malt. Darker malts were invented to keep beers at accustomed colors once malt kilning improved.

I can post my references, if people are interested.

I entered the OG/FG wrong, don't know why. OG has been around 1.05 to 1.055, FG 1.01 to 1.012.